1. Field of the Invention
This invention relates generally to indirectly heated cathodes for electron tubes and is concerned more particularly with an indirectly heated cathode for an electron gun in a cathode ray type tube.
2. Discussion of the Prior Art
An electron tube of the cathode ray type may comprise a generally funnel-shaped envelope having a neck-end portion wherein an electron gun is axially disposed for directing an electron beam onto an anode imaging screen adjacent the large diameter end of the envelope. The electron gun includes a thermionic cathode which generally is of the indirectly heated type comprising an inverted cathode cup having a closed end outer surface provided with an electron emissive coating. Axially disposed within the cathode cup is an electrically insulated heater coil of filamentary wire which is heated by an electrical current flowing through the coil. Thus, a predetermined value of current may be passed through the coil to heat it to a suitable temperature for radiating sufficient thermal energy to the adjacent walls of the cathode cup to heat the electron emissive coating to a desired electron emitting temperature.
The cathode cup may be insulatingly supported within an inverted first grid cup having a closed end provided with a central aperture which is disposed in close-spaced alignment with the electron emissive coating. Therefore, electrons emitted from the electron emissive coating are directed toward the aperture in the first grid and are permitted to pass through it in accordance with the electrical bias potential of the first grid with respect to the potentia1 of the cathode cup. Accordingly, the first grid generally is biased at a steady-state value of potential suitable for permitting passage of a predetermined value of electron current through the aperture in the first grid cup. Also, the first grid may have applied to it a variable signal voltage which alters its bias potential for producing corresponding increases and decreases in the predetermined value of electron current passing through the aperture in the first grid cup.
The electron gun also may include an axial series of spaced beam-forming electrodes which are aligned with the first grid for forming electrons passing through the aperture in the first grid into a beam and focussing the beamed electrons to impinge on a circular spot area of the anode imaging screen. Also, the electron beam may be deflected by conventional means to scan over successive circular spot areas of the imaging screen in well-known fashion. Thus, by varying the electron current in the beam as it scans over the successive circular spot areas, there may be produced on the anode imaging screen a visible light image which is viewable through the large diameter end of the tube envelope.
In order to enhance resolution in the visible light image, it is necessary that the beam-forming electrodes of the gun focus the beamed electrons onto a minimized spot area of the anode imaging screen. Consequently, it is desirable that the electron emissive coating on the closed end of the cathode cup be heated to a temperature sufficient for producing a space-charge limited emission of electrons from the coating. Preferably, the space-charge limited emission is distributed over the exposed surface of the electron emissive coating in a substantially bell-shaped configuration with the heaviest emission being from the central portion of the coating. Thus, there will be provided between the respective closed ends of the cathode and the first grid a reserve of electrons adequate for supplying not only the predetermined value of electron current but also increases required in the predetermined value. These increases may be occasioned by corresponding variations in the signal voltage applied to the first grid and adjustments in the steady-state value of the first grid bias potential to cause an increase in brightness in the visible light image produced on the anode imaging screen.
In operation, it may be found that the central portion of the electron emissive coating on the closed end of the cathode cup operates at a significantly lower temperature as compared to outer marginal portions of the coating. This difference in operating temperatures may be due to heat being lost from the central portion by radiation through the aligned aperture in the first grid, whereas heat radiated from outer marginal portions of the coating may be reflected back thereto by aligned portions of the first grid encircling its aperture. As a result, the central portion of the electron emissive coating may provide only a temperature-limited emission of electrons which is inadequate for establishing the desired reserve of electrons between respective closed ends of the cathode and the first grid.
Consequently, when the steady-state value of potential on the first grid is decreased negatively with respect to the potential of the cathode, the additional electrons required for supplying a corresponding increase in electron current are drawn from outer marginal portions of the electron emissive coating. The resulting electron beam impinging on the anode imaging screen produces a halo-like spot of relatively larger size which degrades resolution in the visible light image produced on the imaging screen. Also, when the temperature of the heater coil within the cathode cup is increased to raise the temperature of the central portion of the electron emissive coating, it may be found that component materials of the cathode sublimate and cause electrical leakage which deteriorates tube performance.